Pope Francis marks his 87th birthday.

The pontiff continues his reign as the oldest pope of the last century and one of the oldest popes in Church history. Pope Francis turned 87 on Sunday, capping a year that has seen milestones in his efforts to reform the Catholic Church as well as health problems that raise questions about his future as pope. Francis celebrated his birthday with a cake during a festive audience with children Sunday morning, and posters reading "Happy Birthday" were hung in St. Peter's Square during his weekly midday blessing. According to the online resource Catholic Hierarchy, only seven popes are known to have been older than Francis at the time of his death. Francis is fast approaching one of them, Pope Gregory XII, perhaps best known for being the last Pope to retire until Pope Benedict XVI, who resigned in 2013. Gregory was 88 and a half years old when he voluntarily resigned in 1415 in an attempt to end a Western schism in which three rivals claimed the papacy. Francis has said he, too, would consider resigning if health conditions prevented him from continuing, but more recently he has declared that the office of pope is for life.